Certain instruments, for example well-logging instruments such as pulsed neutron devices and x-ray emitting devices, require the use of very high voltages within relatively small and confined spaces, in the presence of ionizing radiation and typically at high temperatures. In such instruments, the components operated at high voltage are located near ground potential components, such as the instrument housing. The high voltage operated components and the ground potential components are electrically isolated from each other using insulation that can occupy a tightly confined space. FIG. 1 shows schematically an instrument having a high voltage, charge and ionizing radiation source 201, and an insulator 202 on which the charge and ionizing radiation source is mounted. The source and insulator are located in a housing 203, with a gas-insulated gap 204 spacing the source from the housing. The high voltage, charge and ionizing radiation can derive from separate elements within the source.
The source 201 causes ionization events in the insulating gas. In the gas, the high electric field produced by the voltage difference between the source and the housing 203 causes positive ions and electrons to flow in opposite directions; either to the outer surface of the source or to the inner surface of the housing. Destructive and uncontrolled arcing or voltage breakdown 205 between the source and the housing can then result.